1. Field of the Invention
The invention relates to satellite-based position receivers such as GPS (Global Positioning System) or GLONASS (Global Navigation Satellite System) receivers.
2. Discussion of the Background
These systems use a constellation of satellites which rotate around the earth in very precisely determined orbits, that is to say the position of any satellite may be ascertained at any instant. The satellites transmit radio frequency signals containing navigation data and codes which enable each satellite to be identified. These codes phase-modulate (BPSK modulation) a carrier frequency. A receiver, on the ground or on a land, air or sea vehicle, can receive the signals from several satellites simultaneously, accurately calculate its distance from each of the satellites, and thereby deduce its precise position in terms of latitude, longitude and altitude, in a terrestrial reference frame. It may also thereby deduce the date and precise time of reception in the temporal reference frame of the system. Lastly, it may thereby deduce, through Doppler measurements, its own velocity vector in the terrestrial reference frame (the case of a receiver mounted on a mobile vehicle).
The GPS system, like the GLONASS system, uses two distinct radio frequency bands corresponding to a civil application (frequencies L1) and a military application (frequencies L2) respectively. For the GPS system, there is just one frequency L1, equal to 1575.42 MHz and one frequency L2, equal to 1227.60 MHz. For the GLONASS system, there is a different frequency L1 for each satellite of the constellation and, likewise, a different frequency L2 for each satellite; the Glonass band L1 may be regarded, given its current state and future alterations, as extending from 1590 MHz to 1620 MHz. The band L2 extends between 1238 and 1265 MHz.
The detection of the signals from a satellite is carried out by searching for a phase modulation code present in the radio signal. The signals from the satellite are received by an antenna and sent to an analogue circuit which transposes the modulated radio frequency into a lower frequency modulated in the same way, and which converts the transposed signal into digital before sending it to a digital signal processing circuit. It is the digital processing circuit which detects the presence of the code, by correlating it with an identical code generated locally, and which deduces, from the temporal position of the local code, information on pseudo-distances subsequently making it possible to determine the position of the receiver.
It will be understood that the analogue circuit must make it possible to transpose the frequency of the radio signal, without losing the modulation thereof, to a frequency low enough for the transposed signal to be able to be processed in the digital signal processing circuit. The latter is a silicon-based integrated circuit whose working frequency is limited to a few tens of megahertz.
One of the purposes of the invention is to design the internal circuits of the receiver in such a way that it is easy to adapt one and the same circuit to the construction of receivers of different types and different applications.
The expression xe2x80x9ctype of receiverxe2x80x9d is understood to mean that the receiver is customized for the GPS system or customized for the GLONASS system, or else dual-purpose, that is to say capable of receiving the signals from either of the two systems by choice.
The expression xe2x80x9capplicationxe2x80x9d is understood to mean chiefly that the receiver can receive only the signals of the civil frequency band L1 or on the contrary can also receive the signals of the band L2.
It would be desirable, in order to reduce the costs of development of the receivers, for the internal circuits of these receivers to be capable, in the simplest case, of operating solely under the GPS system on the civil frequency L1, or, in the most complicated case, of operating with the bands L1 and L2 of the GPS system and of the GLONASS system. And of course, these circuits ought also to be able to serve for all intermediate applications, for example a purely civil receiver (frequencies L1) which can receive GPS signals and also GLONASS signals.
Digital signal processing circuits can be designed to operate with GLONASS satellites and also with GPS satellites, although the solutions for achieving this are not simple.
The invention relates to the analogue circuit for frequency transposition and for analogue/digital conversion which precedes the digital signal processing circuits. A purpose of the invention is to ease the implementation of the frequency transposition by rendering this implementation as independent as possible of the type of receiver and of the application envisaged.
For reasons to do with protection against the disturbances created by radio signals of any kind which travel through the atmosphere, it is necessary to perform very selective filterings of the signals received, and this then makes it necessary to perform several frequency transpositions. These successive transpositions make it possible to move progressively from the radio frequency to a frequency acceptable in the digital processing circuit.
By way of example, two successive changes of frequency can be used to go from the frequency L1 for GPS (1575.42 MHz) to an intermediate frequency of the order of a few, hundred MHz, and then to a frequency of the order of 20 MHz.
It has already been proposed, in order to reduce the number of local oscillators required to carry out the frequency transposition, in the case of a GPS operating both on the frequency L1 and on the frequency L2, to choose a frequency of around 175 MHz as intermediate frequency, since then the same local oscillator, at a middle frequency between L1 and L2 (around 1400 MHz) can serve to frequency-transpose the signals at frequency L1 and also the signals at frequency L2; the transposed signals can then go through the same circuits (especially the filters) since they are in the same narrow band of frequencies around 175 MHz.
Moreover, as is well known, frequency transposition is carried out in principle by a mixer which receives on the one hand a signal to be transposed and on the other hand the signal from a local oscillator; the local oscillator is based on a pilot oscillator, a frequency divider and a phase-locked loop; one and the same pilot oscillator can then serve to form several local oscillators if several frequency dividers are used.
Lastly, as far as the invention is concerned, t-he final frequency transposition step will be regarded as being performable either in a mixer (as explained above), or in the analogue/digital converter which ultimately prepares the signal destined for the digital signal processing circuit. This is because, when the sampling frequency Fe of a converter is less than twice the central frequency Fc of the spectrum of the signal received by the converter, the phenomenon of spectral aliasing, well known in sampled systems, means that the frequency of the output signal from the converter has a spectrum centered around the difference frequency Fexe2x88x92Fc, this being the equivalent of a frequency transposition from Fc to Fexe2x88x92Fc.
According to the invention there is proposed an analogue circuit for receiving satellite signals, comprising frequency transposition circuits and an analogue/digital converter, characterized in that the analogue circuit comprises, in order to carry out the frequency transposition, at least two frequency dividers, of which the first is programmable so as to provide at least the following division ratios: 140 and 143.
It has been found that these ratios offer noteworthy possibilities of being able to freely choose, without changing circuit and with simple programming, the type of receiver and the desired application thereof from among several different types and applications.
The programming is particularly easy if the divider possesses an electrical control input for the division ratio so as to provide the chosen ratio as a function of the signal set up on this input. A simple control bit makes it possible to choose the value 140 or 143.
Preferably, the second divider then has a division ratio equal to 10 and there is a third divider with ratio 3. However, the third divider is preferably also programmable so as to make it possible to choose, preferably via electrical control, at least one of the following three ratios: 3, 5, or 7.
To broaden the range of possible applications without changing the structure of the analogue circuit, arrangements are preferably made for the first divider to be programmable so as to have the values 140, 143 and 137. For further broadening, it may be programmable so as to provide a fourth value 142. However, the first divider may also be programmable so as to provide all the integer values between 137 and 143, or even between 136 and 143.
The third divider may be programmable so as also to provide a division ratio of 8.
Lastly, the second divider may be programmed so as to provide the value 10 or the value 11, but it is not necessary to make provision for a separate control in respect of this divider, the control of the third divider being sufficient, associated with a very simple decoder, to determine the ratio of the second divider as a function of the ratio imposed on the third divider: as will be seen, the value 11 for the second divider will in principle be systematically associated with the value 8 for the third.
In practice, provision will preferably be made for the circuit to comprise three dividers, and means for making it possible to obtain at least one of the following combinations of division ratios, where N1, N2 and N3 denote the division ratios of the first, second, and third divider respectively:
These combinations are in fact those which offer, the best practical possibilities of use of the analogue circuit.
Frequency dividers which are programmable may be controlled independently of one another, but it is also possible to make provision, inside the analogue circuit, for a simple decoder, with three input controls, so as to select a combination of three division ratios out of eight possible combinations corresponding to particular predetermined uses of the receiver. The decoder then has outputs for separately controlling the various dividers.